Driving engines of motor vehicles, thus, especially internal combustion engines and electric motors, generate a drag torque when the driver abruptly removes his foot from the accelerator pedal or when a coupling connecting the driving engine to the drive wheel engages quickly upon downshifting. Depending on how great the acting forces are in this instance, according to Kamm's circle, it may happen that the maximum force-transmission capability of the drive wheel between vehicle and roadway, especially the cornering force, is exceeded. Depending on the type of vehicle, construction of the driving engine and type of drive, the drag torque may therefore have a negative effect on the vehicle behavior. In the worst case, the drag torque may lead to skidding of the drive wheel, so that the motor vehicle becomes unstable.
To avoid this, conventionally, an engine drag-torque control is carried out when the driver takes his foot off the accelerator pedal or moves it to such an extent that the driving engine generates a drag torque. Usually, this control is part of the widely prevalent electronic stability program (ESP) and the traction control system (TCS). The engine drag-torque control prevents the drive wheel(s) from skidding, especially on a slippery roadway, by increasing the engine speed short-term, if necessary, in order to keep the vehicle stable and steerable.
Accordingly, a positive drive torque is predetermined for the driving engine, even though a drag torque is called for, in order to prevent the vehicle from becoming unstable. The condition of becoming unstable may be ascertained in conventional manner with the aid of the electronic stability program or the traction control system. For example, depending on the type of vehicle, a maximum engine drag torque is usually preset, which must not be exceeded in order to avoid skidding of the drive wheel(s).